The invention relates to a modified fluororesin comprising a functional group-containing organic compound grafted onto the surface of a crosslinked fluororesin, and more particularly to a modified fluororesin which has been improved in ion-exchange property, hydrophilicity, adhesive property, abrasion resistance or other properties by graft copolymerizing a specific side-chain monomer onto a backbone polymer in a crosslinked fluororesin.
Among fluororesins, tetrafluoroethylene polymers (hereinafter referred to as xe2x80x9cPTFExe2x80x9d), tetrafluoroethylene-perfluoro (alkyl vinyl ether) copolymers (hereinafter referred to as xe2x80x9cPFAxe2x80x9d), and tetrafluoroethylene-hexafluoropropylene copolymers (hereinafter referred to as xe2x80x9cFEPxe2x80x9d) are known as radiation-degradable resins. The mechanical strength of these resins is known to be significantly lowered upon exposure to a very small quantity of an ionizing radiation to such an extent that the resins no longer can be used as materials. For example, in the case of PTFE, upon exposure to a xcex3 radiation in air at a radiation dose of 5 kGy, the mechanical strength at break is reduced to not more than 10 MPa and the elongation is reduced to not more than 100%, and, upon exposure to the xcex3 radiation in vacuo at a radiation dose of 15 kGy, the mechanical strength at break is reduced to not more than 15 MPa and the elongation is reduced to not more than 100%.
In general, grafted resins cannot be put to practical use when resins used as the base have low mechanical strength. Therefore, when a functional group-containing radiation-graftable organic compound (a functional monomer) is grafted onto the above fluororesins by the application of an ionizing radiation, the mechanical strength of the resins is lowered unless a graft reaction takes place at a very low radiation dose of about 10 kGy. Thus, in the conventional radiation grafting of fluororesins, the mechanical strength of the resin is incompatible with the graft level of the functional organic compound. That is, when the radiation dose is decreased, the graft reaction becomes unsatisfactory and, consequently, the properties of the functional group cannot be satisfactorily provided. On the other hand, when the radiation dose is increased to a level which causes a satisfactory graft reaction, the mechanical strength and elongation of the fluororesin as the base resin are lowered, and, consequently, the grafted fluororesin cannot be put to practical use.
Accordingly, it is an object of the invention to provide a modified fluororesin which has satisfactory mechanical strength and, at the same time, has, imparted thereto, for example, satisfactory ion-exchange capacity, hydrophilicity, adhesive property, or abrasion resistance, and a process for producing the same.
According to the first feature of the invention, a modified fluororesin comprises: a crosslinked fluororesin produced by exposing a fluororesin at a temperature at or above the melting point of the resin to an ionizing radiation to crosslink the fluororesin; and a functional group-containing organic compound which has been grafted onto the crosslinked fluororesin by ionizing radiation irradiation. Thus, the modified fluororesin according to the invention comprises a functional group-containing organic compound which has been radiation grafted onto a crosslinked fluororesin. In the modified fluororesin according to the invention, various properties could have been imparted to the fluororesin without sacrificing the mechanical properties of the fluororesin, and a tensile strength at break of not less than 10 MPa and an elongation of not less than 50% can be realized. Here the tensile strength at break and the elongation were measured according to JIS K 7161 using an IA-type test piece specified in JIS K 7162 at a tensile speed of 200 mm/min.
According to the second feature of the invention, a process for producing a modified fluororesin comprises the steps of: exposing a fluororesin heated at a temperature at or above the melting point of the resin in an inert gas atmosphere having an oxygen concentration of not more than 10 Torr to an ionizing radiation at a radiation dose of 0.1 kGy to 10 MGy to prepare a crosslinked fluororesin; exposing the crosslinked fluororesin to an ionizing radiation at a radiation dose of 10 kGy to 5 MGy; and then bringing the crosslinked fluororesin into contact with a functional group-containing organic compound to cause a graft reaction.
According to the third feature of the invention, a process for producing a modified fluororesin comprises the steps of: exposing a fluororesin heated at a temperature at or above the melting point of the resin in an inert gas atmosphere having an oxygen concentration of not more than 10 Torr to an ionizing radiation at a radiation dose of 0.1 kGy to 10 MGy to prepare a crosslinked fluororesin; and exposing the crosslinked fluororesin to an ionizing radiation at a radiation dose of 10 kGy to 5 MGy in the presence of a functional group-containing organic compound to cause a graft reaction.
Preferred embodiments of the invention will be described. The above-described PTFE, PFA, and FEP may be mentioned as fluororesins usable in the invention. The form of the fluororesin is not particularly limited, and examples thereof include particles, sheets, films, blocks, and fibers. Further, a laminate or a composite formed of two or more of these materials or a laminate or a composite formed of at least one of these materials and other material(s) may also be used.
The above-described PTFE embraces those containing not more than 1% by mole of polymer units based on a comonomer, such as perfluoro(alkyl vinyl ether), hexafluoropropylene, (perfluoroalkyl)ethylene, or chlorotrifluoroethylene. In the case of the fluororesin in a copolymer form, a minor amount of a third component may be contained in the molecular structure.
The crosslinked fluororesin according to the invention may be produced by exposing a fluororesin heated at a temperature at or above the melting point of the fluororesin in an inert gas atmosphere having an oxygen concentration of not more than 10 Torr to an ionizing radiation at a radiation dose of 0.1 kGy to 10 MGy. When the oxygen concentration of the atmosphere exceeds 10 Torr, the crosslinking effect is unsatisfactory. When the radiation dose of the ionizing radiation is less than 0.1 kGy, the crosslinking effect is unsatisfactory, while when the radiation dose exceeds 10 MGy, the elongation or the like is significantly lowered. The crosslinked fluororesin may be produced by exposing a sheet or a block of a fluororesin to an ionizing radiation. Alternatively, the crosslinked fluororesin may be produced by molding a fluororesin powder, which has been exposed to an ionizing radiation, for example, by compression molding into a sheet or a block.
Ionizing radiations usable in crosslinking the fluororesin include xcex3 radiation, electron beams, X radiation, neutrons, and high energy ions. In applying the ionizing radiation, the fluororesin should be previously heated at a temperature at or above the crystalline melting point of the fluororesin. For example, when PTFE is used as the fluororesin, the fluororesin should be exposed to an ionizing radiation in such a state that the fluororesin has been heated to a temperature of 327xc2x0 C. (the crystalline melting point of this material) or above. When PFA is used, this fluororesin is exposed to an ionizing radiation in such a state that the fluororesin has been heated to a temperature of 310xc2x0 C. (the crystalline melting point of this material) or above. When FEP is used, this fluororesin is exposed to an ionizing radiation in such a state that the fluororesin has been heated to a temperature of 275xc2x0 C. (the crystalline melting point of this material) or above. Heating the fluororesin at a temperature at or above the melting point of the fluororesin can energize the molecular motion of the backbone constituting the fluororesin and consequently can efficiently accelerate an intermolecular crosslinking reaction. Excessive heating causes cleavage and decomposition of the molecular backbone. The G upper limit of the heating temperature should be 10 to 30xc2x0 C. above the melting point of the fluororesin from the viewpoint of inhibiting the occurrence of this depolymerization phenomenon.
The modified fluororesin according to the invention may be produced by grafting a functional group-containing organic compound onto the above crosslinked fluororesin by the application of an ionizing radiation. Grafting methods using a radiation are classified into a pre-irradiation method, wherein a radiation is previously applied to the backbone polymer of the fluororesin to produce radicals as an origin of grafting and the fluororesin is then brought into contact with a functional group-containing organic compound, and a simultaneous irradiation method wherein an ionizing radiation is applied to the fluororesin in the presence of the functional group-containing organic compound. Any of these methods may be used in the invention.
Ionizing radiations usable herein include radiations or ions of not less than 100 keV, such as xcex3 radiation, electron beams, X radiation, and protons, which can permeate the fluororesin by 10 to 100 xcexcm or more. High-energy plasma may also be used. The radiation dose of the ionizing radiation is preferably 10 kGy to 5 MGy. When the radiation dose is less than 10 kGy, the effect of a graft reaction on a level such that the properties of the functional group can be effectively allowed to function, is less likely to be attained. On the other hand, when the radiation dose exceeds 5 MGy, the elongation or the like is likely to be significantly lowered. When the temperature at the time of the ionizing radiation irradiation is high, the disappearance of radicals takes place. Therefore, the temperature at the time of the ionizing radiation irradiation is preferably room temperature or below. The application of the ionizing radiation may be carried out in an inert gas atmosphere, or alternatively may be carried out in the presence of oxygen.
The functional group-containing organic compound to be graft polymerized onto the crosslinked fluororesin may be properly selected according to the properties to be imparted to the fluororesin. For example, when imparting the ion-exchange capacity is contemplated, an organic compound containing a phenolic hydroxyl group, a carboxylic acid group, an amino group, a sulfonic acid group or the like is used. Further, an acyloxy group, an ester group, and an acid imide group can be quantitatively converted to a phenolic hydroxyl group, a sulfonic acid group or the like by hydrolyzing the grafted product. Therefore, organic compounds containing these functional groups may also be used when imparting the ion-exchange capacity to the fluororesin is contemplated. Specific examples of organic compounds having an ion-exchange functional group include organic compounds having an ion-exchange functional group, such as hydroxystyrene, acyloxystyrene, acrylic ester, methacrylic ester, maleic ester, fumaric ester, vinylester, a vinylamine compound, vinylpyridine, vinylsuccinimide, and vinylsulfonic ester.
In the case of acyloxystyrene, a substituent may be located at the o-, m-, or p-position, and the hydrocarbon group contained in the acyl group is preferably a straight-chain or branched aliphatic, alicyclic, or aromatic hydrocarbon having 1 to 15 carbon atoms. p-Acetoxystyrene is most commonly used. In the case of esters such as acrylic esters and methacrylic esters, the hydrocarbon group contained in the ester group is preferably a straight-chain or branched aliphatic, alicyclic, or aromatic hydrocarbon group having 1 to 20 carbon atoms.
Organic compounds usable in imparting hydrophilicity include hydroxystyrene, 2-hydroxymethyl methacrylate, 2-hydroxyethyl methacrylate, vinylamine compounds, vinylsuccinimide, vinylsulfonic acid, and vinyl alcohol. The contact angle of water on PTFE is generally 100 degrees. On the other hand, the contact angle of water on the modified PTFE, to which hydrophilicity has been imparted according to the invention, has been found to be in the range of 10 to 80 degrees, indicating that a very high level of hydrophilicity was imparted.
Organic compounds usable in imparting an adhesive property include glycidyl acrylate, glycidyl methacrylate, allyl glycidyl ether, vinyl-containing carbamic ester, and acrylonitrile. When the adhesive property has been imparted in this way, the fluororesin can be easily adhered to metals and other plastics. In this case, suitable metals include stainless steel, steel, aluminum, chromium, nickel, iron, tin, zinc, lead, and manganese. Preferably, these metals are previously subjected to conventional surface treatment, for example, oxidation using an acid solution. However, the modified fluororesin can also be adhered to metals not subjected to the surface treatment. Suitable plastics include polyolefins, such as polyethylene and polypropylene, acetate, vinyl chloride, polystyrene, polyester, polycarbonate, and polyamide.
Organic compounds usable in imparting abrasion resistance include vinylsiloxane, chlorotrifluoroethylene, vinyl chloride, vinylidene chloride, fluorostyrene, and chlorostyrene.
The functional group-containing organic compound may be dissolved in an organic solvent to prepare a solution which is then used in the radiation graft reaction. This organic solvent is preferably one which can homogeneously dissolve the organic compound and does not dissolve the fluororesin. Examples of organic solvents usable herein include: ketones, such as acetone and methyl ethyl ketone; esters, such as ethyl acetate and butyl acetate; alcohols, such as methyl alcohol, ethyl alcohol, propyl alcohol, and butyl alcohol; ethers, such as tetrahydrofuran and dioxane; aromatic hydrocarbons, such as N,N-dimethylformamide, N,N-dimethyl acetamide, benzene, and toluene; aliphatic or alicyclic hydrocarbons, such as n-heptane and cyclohexane; and a mixed solvent composed of two or more of the above organic solvents. Among them, organic solvents, which can swell the crosslinked fluororesin, are preferred.
After the radiation graft reaction, the crosslinked fluororesin may be optionally washed with an organic solvent. Examples of organic solvents usable herein include: alcohols, such as methanol, ethanol, and propyl alcohol; ketones, such as acetone and methyl ethyl ketone; aromatic hydrocarbons, such as benzene and toluene; and a mixture of two or more of the above organic solvents.
When the modified fluororesin is used as an ion-exchange membrane, for example, the acyloxy or ester group as the functional group should be converted by hydrolysis to a hydroxyl group or an acid group. As with the conventional hydrolysis of acyl or ester group commonly carried out in the art, this hydrolysis is very easy as compared with the hydrolysis of the primary alcohol ester, and can be easily carried out under mild conditions. Specifically, an acid or a base is provided as a catalyst, and the grafted crosslinked fluororesin is placed in an aqueous solution containing the catalyst or a mixed solution, composed of water and a water-soluble organic solvent, containing the catalyst to cause the hydrolysis reaction. Since the hydrolysis reaction is mainly carried out in a heterogeneous system, the hydrolysis is preferably carried out in a mixture of a water-soluble organic solvent, such as an alcohol or a ketone, with water from the viewpoints of enhancing the affinity of the reactive group for the catalyst and, when an acid catalyst is used, permitting a left organic acid to be dissolved. The temperature of the hydrolysis is suitably 50 to 100xc2x0 C.
According to the invention, the modified fluororesin may be in the form of a sheet, a film, a block, or a fiber as a molded product of the modified fluororesin per se. Alternatively, the modified fluororesin may be in the form of a sheet, a film, a block, or a fiber as a molded product of a compound of the modified fluororesin with other polymer incorporated therein. Other polymers usable herein include unmodified fluororesins, such as unmodified PTFE, unmodified PFA, and unmodified FEP, engineering plastics, such as polyether sulfone and polyimide, and thermoplastic resins, such as polyethylene and polypropylene.